Die casting machine with reduced static injection pressure

ABSTRACT

A die casting method initially injects an alloy with minimal volumetric contraction during solidification into a die cavity, monitors at least one of the pressure of the shot cylinder and the position of the plunger, and reduces the injection pressure during the final stage of filling of the die mold. A die casting machine includes a shot cylinder having one of a pressure detector located for detecting the hydraulic pressure applied to the cylinder or a position sensor for a plunger rod. The plunger rod includes a tip extending into a cold chamber, which receives a metal alloy having a minimal shrinkage characteristic. A source of hydraulic pressure is coupled to the shot cylinder by a control valve and a control circuit is coupled to said valve and to one of said detector or sensor for reducing the injection pressure near the end of an injection cycle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) on U.S.Provisional Application No. 60/879,000, entitled DIE CASTING PROCESS,filed Jan. 5, 2007, by James A. Yurko, et al., the entire disclosure ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to die casting machines and methods forcontrolling the injection pressure, particularly during the final stagesof the injection process. Die casting machines inject metals, polymers,or other material in a controlled fashion into a mold (a.k.a. tool ordie) that is clamped in a closed position by the machine. The metal istypically injected into a die using a hydraulic cylinder. For mostmetals, the metal is injected into the die with a controlled orpredetermined velocity and/or pressure. Back-pressure from pushing themetal through a thin die entrance (i.e., gate) requires significanthydraulic force to overcome such resistance to flow. At the end of diefilling, the hydraulic force of the injection cylinder applieshydrostatic pressure to the metal in the die. During solidification ofthe metal in the die, the metal undergoes a volumetric change thattypically contracts the metal, causing porosity in the part known asshrinkage. Shrinkage is minimized through the injection of more moltenmetal via the high pressure applied to the injection cylinder.Frequently, especially in the casting of aluminum alloys, a higherpressure source is actuated on the head side of the injection cylinder,to further increase the force of the cylinder by a factor of up to fivetimes the injection force used for the initial die filling.

When the cavity is completely filled during injection, and the cylinderapplies force on the solidifying metal, the transferred pressure on themetal counteracts the clamped die pre-load force. When the hydrostatictransmitted force of the metal exceeds the clamping force, the die opensand the molten metal will be ejected under high pressure from the dieresulting in flash. Flashing causes major process problems including: 1)variation in part size and dimensions, 2) damage to the die, and 3)frequent process stops to remove the sticking flash from the die.

The projected area of the casting, that is the surface area of thecasting that is perpendicular to the closing axis of the die castingmachine, is limited by the hydrostatic metal pressure of the solidifyingmetal. The product of the projected area and hydrostatic metal pressurecannot exceed the clamping force of the die casting machine. Forexample, a part with 100 sq. in. of projected area and 10,000 psi ofapplied metal pressure from the injection cylinder would have 1,000,000pounds of separating force, or 500 tons. The 500 tons of force requiresa die casting machine with 500 tons of clamping force to maintain thedie closed during the casting process. This product of hydrostatic metalpressure and casting projected area constrains the size of parts thatcan be produced for a given size of die casting machine.

FIG. 1 shows a typical molten metal injection system including ahydraulic shot cylinder 10 having a piston 12 coupled to a plunger 14.The diagram represents part of an overall die cast machine which can beof a conventional commercially available design. Shot cylinder 10 isactivated by a pressurized source of hydraulic fluid applied at an inlet16. An outlet 18 releases hydraulic fluid from the shot cylinder to areservoir 19 through valve 17. Plunger 14 extends into a cylindricalcold chamber 20, which has a molten metal inlet 22. Plunger 14 has aplunger tip 24, which typically has a smaller diameter than the diameterof the shot cylinder piston 12. Plunger tip 24 forces molten metal outof an exit end 26 of cold chamber 20. The exit end 26 of cold chamber 20communicates with one or more runners 32 formed in die halves 30, 31.The width of runner(s) 32 is typically less than 1 inch. The runner(s)32, in turn, each communicate with a gate 34 leading to the mold cavity36 in die halves 30, 31. The mold cavity 36 typically will alsocommunicate with one or more overflow cavities 38.

When the die cavity 36 is completely filled during injection, and theshot cylinder 10, applies force on the solidifying metal, thetransferred pressure on the metal counteracts the clamped die pre-loadforce. If the hydrostatic transmitted force of the metal exceeds theclamping force, the die opens and the metal will flash. Thus, flashingoccurs when molten metal is ejected under high pressure between the diehalves and can cause major process problems including a variation inpart size and dimensions, damage to the die, and frequent process stopsto remove the sticking flash from the die halves before the nextinjection cycle.

An example of a typical injection profile for the operation of themachine of FIG. 1 is shown in FIG. 2. The available force of the shotcylinder 10 is plotted versus the position of the plunger rod 14. Inregime 1, the hydraulic cylinder pushes the molten metal in the coldchamber 20 towards the cavity 36. Typically, the cold chamber is notcompletely filled after pouring, so in this flow regime, an injectioncylinder advances the metal to completely fill the cold chamber 20.There is little resistance to this fluid flow because the cold chamberis typically a cylinder with large diameter (greater than 1″). At theend of regime 1, when the cold chamber is filled, the separating forceis now equal to the metal pressure in the cold chamber multiplied by thecross-sectional area of the cold chamber.

In regime 2, the runner 32 begins to fill with metal. The runner issubstantially smaller in cross section than the cold chamber, typicallyless than 1″ in diameter. The smaller cross section begins to createback pressure in the metal within the runner and cold chamber, and thusthe hydraulic fluid in shot cylinder 10. At the end of regime 2, theseparating force increases by an amount equal to the metal pressuremultiplied by the projected area of the runners plus the increasedpressure applied to the cross-sectional area of the cold chamber.

At the end of regime 2 and the beginning of regime 3, the metal beginsto flow into the part through the gate 34. The gate is relatively thin,with a thickness which can vary from 0.020″ to 0.500″, but is typicallyless than 0.100″ for most die castings.

During regime 3, metal pressure in the cavity rises from resistance toflow through thin sections of the part in mold cavity 36. The metalpressure in the cavity now begins to transmit onto the closed die halves(30, 31), which are held closed by the clamping force of the die castmachine. The hydraulic cylinder pressure in the head side of the shotcylinder also rises. Resistance to flow is not yet maximized because themetal can still flow within the cavity 36 and also into the overflows38. If intensification (an increase in the force of the shot cylinder)is utilized, it is typically triggered during the final stages of regime3. Intensification is essential for alloys such as aluminum that undergovolumetric shrinkage. By the end of regime 3, the die separating forcehas risen substantially.

In regime 4, the final sections of the mold cavity 36 and overflows 38are filled.

Overflows are designed to create back pressure within the casting, andalso capture metal ridden with gas, lubricants, defects, etc. Thepressure rises yet again within the metal and the hydraulic cylinder,and the die separating force correspondingly increases because of theincreased back pressure on the metal in the casting portion of thecavity.

In regime 5, the casting and overflows are completely filled, and thehydrostatic force in the metal rises to its maximum value. For asolidifying metal, the force peaks in the initial stages when the metalremains almost completely a fluid. When the metal begins solidifying,the pressure is only hydrostatically transferred to the projected areaon the die in contact with molten metal. The metal pressure during thisstage of the casting cycle, combined with the molten projected area ofthe casting, typically dictates the necessary clamping force andtherefore size of the die casting machine. For metals that undergovolumetric shrinkage, the hydraulic cylinder may advance some distancebecause additional molten metal within the runner will enter the partcavity into the void created by the shrinking, solidifying metal. If theseparating force in regime 5 exceeds the die clamping force, the diewill flash, creating significant process related problems noted above.

Recent advances in materials technology have led to the development ofalloys that experience minimal or no volumetric contraction duringsolidification or cooling. An example of this is a metallic glass alloy,such as described in U.S. Pat. Nos. 5,618,359 and 7,017,645, thedisclosures of which are incorporated herein by reference. These alloysare viscous and require a relatively high injection force to push themetal through the gate(s) and fill the cavity. However, because there islittle or no volumetric contraction of the solidifying material(shrinkage), the applied static force of the hydraulic cylinder when thecavity is full only serves to place limits on the size of part that canbe made for a given die casting machine. Therefore, while a high dynamicforce is required to fill the die, a high static force only serves tolimit the projected area of parts that can be made on a given diecasting machine.

SUMMARY OF THE INVENTION

A die casting machine injection system of the present inventiondecreases the hydraulic force during the final stages of injection of amolten alloy into a die, culminating with a final hydrostatic pressureon the alloy in the filled cavity that is less than the dynamicinjection pressure. This is in contrast to current state of the art diecasting machines which use either the same hydraulic force for injectionand intensification, or a higher intensification force and do notdecrease force during the final stages of injection and intensification.

A method of die casting parts according to the present inventionincludes initially injecting a molten alloy having the characteristicsof minimal volumetric contraction during solidification into a diecavity during a filling stage, monitoring at least one of: 1) thepressure of the shot cylinder for injecting a molten alloy into the diecavity or 2) the position of the plunger rod for injecting the moltenalloy into a die cavity, and reducing the injection pressure during thefinal stage of filling of the die mold for reducing the die separatingforce at the end of a molding cycle.

A die casting machine embodying the present invention includes a shotcylinder having one of a pressure detector located for detecting thehydraulic pressure applied to the cylinder or a position sensor for aplunger rod. The plunger rod includes a tip extending into a coldchamber, which receives a molten alloy having a minimal shrinkagecharacteristic. The machine also includes a source of hydraulicpressure, a control valve coupling said hydraulic pressure source tosaid shot cylinder, and a control circuit coupled to said valve and toone of said detector or sensor for reducing the injection pressure nearthe end of an injection cycle.

The resultant machine and operation greatly reduces the die separatingforces at the end of a casting cycle where low shrinkage alloys areemployed and allows the casting of larger parts with lower tonnage diecasting machines. Larger projected areas of parts can be made on thesame size of die casting machine that was previously limited by thehigher final force. This increases the size and/or number of castingsthat may be made on a given die casting machine. The cost of the diecasting machine can also decrease, because less clamping force isnecessary to perform the process for a given sized part.

These and other features, objects and advantages of the presentinvention will become apparent upon reading the following descriptionthereof together with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the injection system of a typicalmolten metal die casting machine;

FIG. 2 is a diagram showing an injection profile (force verses stroke)of such a typical die casting machine;

FIG. 3 is a schematic diagram of the injection system of a die castingmachine incorporating the present invention;

FIG. 4 is a diagram showing an injection profile (force verses stroke)of the method of operation of the machine shown in FIG. 3; and

FIG. 5 is an electrical diagram in schematic and block form of thecontrol system for the machine shown in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In order to control the final pressure when molding an alloy which doesnot exhibit the characteristics of shrinking during solidification,i.e., an alloy such as a metallic glass alloy, the injection systemshown in FIG. 3 and its method of control can be employed. It should beunderstood that the injection system of FIG. 3 is part of an overall diecasting machine, which can be of the type disclosed in U.S. PublishedPatent Application 2003/0217829, the disclosure of which is incorporatedherein by reference. The injection method and equipment of FIG. 3,however, can be incorporated in any conventional die casting machine toachieve the desired result of reducing the final injection pressureduring the casting process.

In FIG. 3, an injection system 60 is disclosed which includes a shotcylinder 70 supplied on its intake side through an inlet 72 coupled to acontrol servo valve 74 and, in turn, coupled to a source of hydraulicfluid pressure 76. The net forward force provided by shot cylinder 70can be reduced by decreasing the head-side pressure in the shotcylinder. A servo valve 74 (FIGS. 3 and 5), under the control of circuit50, can be used to reduce the pressure during the final stages offilling from a high pressure source of hydraulic fluid 76, reducing thehead pressure and, thus, the net forward force.

Shot cylinder 70 includes a piston 80 and plunger rod 82 extendingtherefrom having a plunger tip 84 which extends into a cold chamber 86coupled to die halves 90 and 91. Cold chamber 86 has an inlet 88 throughwhich molten alloy, such as a glass metal alloy is poured for fillingthe cold chamber 86 prior to the injection molding of the alloy into adie cavity 96 through gate 94 and outlet 89 of cold chamber 86. Diecavity 96 also communicates with an overflow 98. Cavity 96 forms, withdie halves 90 and 91, the shape of a part to be molded. Restricting thehydraulic flow out of the rod-side outlet 71 of the shot cylinder 70 isa technique for controlling velocity of the shot cylinder plunger rod82. A servo-hydraulic, flow-control valve 73 (FIGS. 3 and 5), such asthe Parker TDL valve, controls the amount of hydraulic fluid exiting therod-side of the shot cylinder into a reservoir 75. Restricting thisexiting flow raises the pressure in the rod-side hydraulic fluid, alsodecreasing the net forward force of the shot cylinder. Theservo-hydraulic valve 73 can completely restrict the flow from therod-side of the shot cylinder, thus stopping the shot cylinder anddecreasing its net forward force to zero. The servo-hydraulic valve canbe controlled by different techniques; two examples include shifting thevalve at a predetermined position of the shot cylinder based upon aninput signal from a position sensor 54 (FIG. 5) associated with rod 82or shifting the valve when the pressure (detected by pressure detector52 shown in FIG. 5) rises above a selected level that is associated withthe end of the part-cavity filling, regime 3 of FIG. 4. The FIG. 4example of an injection profile using the invention shows the largeinjection forces in regime 3 necessary to inject the molten alloythrough the thin orifice (gate) into the casting, which itself is quitethin. The available force is shown by the dotted line. However, when thecasting is nearly filled, the force is reduced (dash-dot line). Theforce cannot be decreased too early in the process, otherwise thecasting will not completely fill with molten alloy. One major benefit ofthe process is the reduced die separating force in regime 5, especiallywhen compared with that of FIG. 2, regime 5.

A bypass conduit 100 couples outlet 71 to inlet 72 by means of a controlvalve 102 for further controlling the force applied by the plunger rod82 to plunger tip 84. As shown in FIG. 5, the control circuit 50 hasoutputs which provide signals to control servo valve 74, bypass valve102, and discharge valve 73. Circuit 50 receives pressure informationfrom detector 52 and rod 82 position information from sensor 54. Circuit50 is programmed to control the pressure on piston 80 of shot cylinder70 for controlling the movement of plunger rod 82 and, therefore, thepressure applied to the metallic alloy in cold chamber 86 by plunger tip84 according to the desired characteristics as shown in the pressureprofile of FIG. 4. Each specific part defined by mold cavity 96 mayrequire a specific pressure profile also the use of different glassmetal alloys may result in different operating pressure profiles. Onetypical profile is illustrated in FIG. 4. Because the flow is restrictedout of the rod-side outlet 71 of the shot cylinder 70, the net force ofthe cylinder is near zero, as shown at regime 5 in FIG. 4.

The ratio of the pressure on the head and rod side of the shot cylinderis inversely proportional to the area of the head and the annular areaon the rod-side of the cylinder. For example, a shot cylinder with a 4″diameter head and a 2″ diameter rod would have a head area of 12.6 sq.in. and an annular area of 9.4 sq. in. Therefore, if the head pressurewas 3000 psi, the rod-side pressure would be 4000 psi. To limit theincrease in pressure on the rod-side of the cylinder, a 1:1 head toannular area shot cylinder can be utilized. Furthermore, the 1:1 ratiocylinder could be utilized to decrease net forward force to zero by notonly completely restricting the flow-out of the rod-side, but also byallowing hydraulic fluid to flow between the head and rod side of thecylinder by opening bypass valve 102 in the coupling (bypass) circuit100. The pressure on the head and rod side are, therefore, equal, knownas a regenerative mode. In this regenerative mode, if the head-side ofthe cylinder has a larger area than the rod annular area, there is a netforward force (a reduced force compared to the capability of thecylinder), and with a 1:1 head to annular area cylinder, the net forwardforce is zero.

A shot cylinder will also have a net-forward force of zero if thecylinder has fully extended. The stroke of the shot cylinder piston 80of the die casting machine can be controlled to reach its limit(detected by a limit sensor) during the overflow filling regime ofcavity fill thus stopping the shot cylinder.

Each of these techniques can decrease the force of the cylinder,anywhere from 0-100% of the dynamic force. The important criteria is todecrease the force within a short period time, on the order of 10 ms orless, or to stop the injection after the part is filled, but theoverflows and therefore the complete cavity have not yet filled. Inorder to increase the time window of decreasing force and/or decreasingvelocity, the overflows can be designed to be filled over a longer timeframe.

It will become apparent to those skilled in the art that variousmodifications to the preferred embodiment of the invention as describedherein can be made without departing from the spirit or scope of theinvention as defined by the appended claims.

1. An injection system for a die casting machine comprising: a shotcylinder having a plunger rod and a hydraulic fluid inlet for receivingpressurized hydraulic fluid for moving said plunger rod; one of apressure detector located for detecting the hydraulic pressure appliedto said shot cylinder and a plunger rod position sensor for determiningthe position of said plunger rod; a cold chamber with an inlet forreceiving a molten alloy having a low shrinkage characteristic, saidplunger rod extending into said cold chamber for injecting a moltenalloy into a die cavity; a source of hydraulic pressure for coupling tosaid shot cylinder; a control valve coupling said hydraulic pressuresource to said inlet of said shot cylinder; and a control circuit havingan output coupled to said valve and an input coupled to at least one ofsaid detector and sensor and responsive to input signals for providing acontrol signal to said valve for reducing the injection pressure appliedby said plunger rod near the end of an injection cycle.
 2. The injectionsystem as defined in claim 1 wherein said shot cylinder includes ahydraulic fluid outlet and a second valve coupled to said outlet,wherein said second valve is coupled to said control circuit andresponsive to signals therefrom for reducing the injection pressureapplied by said plunger rod near the end of an injection cycle.
 3. Theinjection system as defined in claim 1 wherein said shot cylinderincludes a hydraulic fluid outlet and a bypass conduit coupled betweensaid inlet of said shot cylinder and said outlet and a bypass valvecoupled in the bypass conduit, said bypass valve coupled to said controlcircuit and responsive to signals therefrom for reducing the injectionpressure applied by said plunger rod near the end of an injection cycle.4. The injection system as defined in claim 3 and including a secondvalve coupled to said outlet, wherein said second valve is coupled tosaid control circuit and responsive to signals therefrom for reducingthe injection pressure applied by said plunger rod near the end of aninjection cycle.
 5. The injection system as defined in claim 1 whereinsaid molten alloy is a metal glass alloy.
 6. A method of die castingparts comprising the steps of: injecting a molten alloy having thecharacteristics of minimal volumetric contraction during solidificationinto a die cavity during a filling stage; monitoring at least one of thepressure of a shot cylinder for injecting the molten alloy into the diecavity or the position of a plunger rod for injecting the molten metalinto the die cavity; and reducing the injection pressure applied by theshot cylinder and plunger rod during a final stage of filling of the diemold for reducing the die separating force near the end of a moldingcycle.
 7. The method as defined in claim 6 wherein said reducing stepcomprises limiting the injection pressure at the head end of the shotcylinder near the end of said molding cycle.
 8. The method as defined inclaim 7 wherein said limiting step includes providing a valve between ahigh pressure source of hydraulic fluid and the head end of the shotcylinder and controlling the valve to limit the pressure of fluidapplied to the shot cylinder and the resulting injection pressure ofmolten alloy.
 9. The method as defined in claim 7 wherein the shotcylinder has an outlet at a side of a piston opposite the head end andsaid limiting step further comprises providing a valve coupled to theoutlet and controlling the valve to limit the pressure applied by theshot cylinder to the die cavity.
 10. The method as defined in claim 7wherein the shot cylinder has an outlet at a side of a piston oppositethe head end and said limiting step further comprises providing a bypassconduit between the head end of the shot cylinder and the outlet and avalve coupled in the bypass conduit and controlling the valve to controlthe pressure applied by the shot cylinder in injecting molten metal intothe die cavity during a molding cycle.
 11. The method as defined inclaim 6 wherein said molten alloy is a metal glass alloy.
 12. A diecasting machine comprising: a pair of dies defining at least one diecavity for a part to be molded; a clamping system for clamping said diestogether during the injection of a molten alloy into said die cavity; ashot cylinder having a plunger rod and a hydraulic fluid inlet forreceiving pressurized hydraulic fluid for moving said plunger rod; acold chamber with an inlet for receiving a molten alloy having a lowshrinkage characteristic and an outlet coupled to said die cavity, saidplunger rod extending into said cold chamber for injecting a moltenalloy into said die cavity; a source of hydraulic pressure for couplingto said inlet of said shot cylinder; and a control system incommunication with said source of hydraulic pressure for reducing theinjection pressure applied by said plunger rod near the end of aninjection cycle.
 13. The die casting machine as defined in claim 12 andfurther including one of a pressure detector located for detecting thehydraulic pressure applied to said shot cylinder and a plunger rodposition sensor for determining the position of said plunger rod. 14.The die casting machine as defined in claim 13 and further including acontrol valve coupling said hydraulic pressure source to said inlet ofsaid shot cylinder.
 15. The die casting machine as defined in claim 14wherein said control system includes a control circuit having an outputcoupled to said valve and an input coupled to at least one of saiddetector and sensor and responsive to input signals for providing acontrol signal to said valve for reducing the injection pressure appliedby said plunger rod near the end of an injection cycle.
 16. The diecasting machine as defined in claim 15 wherein said shot cylinderincludes a hydraulic fluid outlet and a second valve coupled to saidoutlet, wherein said second valve is coupled to said control circuit andresponsive to signals therefrom for reducing the injection pressureapplied by said plunger rod near the end of an injection cycle.
 17. Thedie casting machine as defined in claim 15 wherein said shot cylinderincludes a hydraulic fluid outlet and a bypass conduit coupled betweensaid inlet of said shot cylinder and said outlet and a bypass valvecoupled in the bypass conduit, said bypass valve coupled to said controlcircuit and responsive to signals therefrom for reducing the injectionpressure applied by said plunger rod near the end of an injection cycle.18. The die casting machine as defined in claim 17 and including asecond valve coupled to said outlet, wherein said second valve iscoupled to said control circuit and responsive to signals therefrom forreducing the injection pressure applied by said plunger rod near the endof an injection cycle.
 19. The die casting machine as defined in claim18 wherein said molten alloy is a metal glass alloy.